Edexcel A Level (IAL) Biology -4.12 Core Practical 9: Antimicrobial Properties of Plants- Study Notes- New Syllabus

CORE PRACTICAL 9 – Investigate the Antimicrobial Properties of Plants

🎯 Aim

To investigate how plant extracts (e.g. garlic, mint, clove, or tea tree oil) affect the growth of bacteria, and to learn safe aseptic techniques for microbiological work.

🔬 Introduction

Many plants produce natural antimicrobial substances that can kill or inhibit bacteria.
This practical test their effectiveness using the agar diffusion method – similar to antibiotic sensitivity testing.

🧪 Materials Required

• Sterile nutrient agar plates
• Non-pathogenic bacterial culture (e.g. E. coli or Bacillus subtilis)
• Plant extracts (garlic, mint, clove, tea tree oil, etc.)
• Sterile paper discs or wells
• Sterile forceps, pipettes
• Marker, adhesive tape, incubator (25°C)
• Disinfectant, gloves, ethanol, and Bunsen burner (for asepsis)

🧍‍♂️ Aseptic Techniques (Safety First!)

Before and during the experiment:

• Disinfect work area before and after use.
• Flame tools like forceps to sterilise before use.
• Keep agar plates covered to prevent contamination.
• Work near a Bunsen flame – air currents reduce airborne microbes.
• Dispose of all used cultures in autoclave-safe bags.

⚗️ Procedure

1. Prepare plant extracts

Crush plant material (e.g. garlic, mint) with a mortar & pestle.
Add sterile water or ethanol → filter to get a clear extract.

2. Prepare bacterial lawn

Pour molten nutrient agar into Petri dish and allow it to set.
Spread bacterial suspension evenly using a sterile swab.

3. Apply plant extracts

Dip sterile paper discs into each extract and place them on inoculated agar (use sterile forceps).
Include a control disc soaked in sterile water or solvent.

4. Incubate

Seal plates lightly with tape (not airtight).
Incubate inverted at 25°C for 24-48 hours.

5. Observe results

Look for clear zones (zones of inhibition) around discs where bacteria were killed or inhibited.
Measure the diameter of each clear zone with a ruler.

📊 Result & Analysis

Larger zone = greater antimicrobial activity.

💡 Conclusion

Some plant extracts contain natural antimicrobial compounds that effectively inhibit bacterial growth.
Garlic and clove usually show the strongest effects – due to allicin (in garlic) and eugenol (in clove).
Effectiveness varies with extract concentration, bacterial strain, and solvent used.

⚠️ Precautions

• Always follow aseptic techniques while handling bacteria.
• Never open plates after incubation.
• Dispose of all cultures using autoclave-safe biohazard waste bags.
• Use only non-pathogenic bacterial strains.

📦 Quick Recap